I'm in 7A with that horrible clay soil too!! All my lavender are in the ground - although with a townhouse I don't have lots of room. I think it would depend how big you want it to get. I've seen lavender flower in 4" pots. I would probably go with a 3-5 gal size though if you want to go the pot route. That should allow them enough room to get a nice size so you can enjoy their fragrance. Good luck!!
I think I was carried away by seeing these huge gorgeous Lavenders in the Oregon desert. I'll move them down a notch in my "must-grow" list. Maybe before I get to them I'll move to a drier climate and/or have some soil that drains worth a darn.
But if I get those Lavender urges again, I'll start them in a one-gallon pot and keep my eyes open for a 3 gallon pot.